Most commercially-available non-silicone hydrogel contact lenses are produced according to a conventional cast molding technique involving use of disposable plastic molds and a mixture of vinylic monomers and crosslinking agents. There are several disadvantages with the conventional cast-molding technique. For example, a traditional cast-molding manufacturing process often includes lens extraction in which unpolymerized monomers must be removed from the lenses by using an organic solvent. Use of organic solvents can be costly and is not environmentally friendly. In addition, disposable plastic molds inherently have unavoidable dimensional variations, because, during injection-molding of plastic molds, fluctuations in the dimensions of molds can occur as a result of fluctuations in the production process (temperatures, pressures, material properties), and also because the resultant molds may undergo non-uniformly shrinking after the injection molding. These dimensional changes in the mold may lead to fluctuations in the parameters of contact lenses to be produced (peak refractive index, diameter, basic curve, central thickness etc.) and to a low fidelity in duplicating complex lens design.
The above described disadvantages encountered in a conventional cast-molding technique can be overcome by using the so-called Lightstream Technology™ (CIBA Vision), which involves (1) a lens-forming composition being substantially free of monomers and comprising a substantially-purified, water-soluble prepolymer with ethylenically-unsaturated groups, (2) reusable molds produced in high precision, and (3) curing under a spatial limitation of actinic radiation (e.g., UV), as described in U.S. Pat. Nos. 5,508,317, 5,583,163, 5,789,464, 5,849,810, 6,800,225, and 8,088,313. Lenses produced according to the Lightstream Technology™ can have high consistency and high fidelity to the original lens design, because of use of reusable, high precision molds. In addition, contact lenses with high quality can be produced at relatively lower cost due to the short curing time, a high production yield, and free of lens extraction and in an environmentally friendly manner because of use of water as solvent for preparing lens formulations.
However, the Lightstream Technology™ has not been applied to make UV-absorbing contact lenses, largely because of the lack of water-soluble photoinitiator which can efficiently initiate curing (polymerization) of an aqueous lens formulation using a visible light having a wavelength from 380 to 460 nm. Examples of known efficient visible-light photoinitiators include 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO), 2,4,6-trimethylbenzoylethoxy-phenylphosphine oxide (TPO-L), and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO), acylgermanium compounds described in U.S. Pat. No. 7,605,190 (herein incorporated by reference in its entirety). But, those available photoinitiators are insoluble in water and cannot be used in the production of contact lenses from an aqueous lens formulation according to the Lightstream Technology™. Some attempts have been made to prepare more hydrophilic phosphine oxide photoinitiators (Majima, Tetsuro; Schnabel, W.; Weber, W. Makromolekulare Chemie 1991, 192(10), 2307-15; De Groot, J. H.; et. al. Biomacromolecules 2001, 2, 1271). The phosphine oxide photoinitiators reported in those studies either have a limited solubility in water or have a much reduced efficiency in initiating polymerization (i.e., prolonging the cure times).
Therefore, there are still needs for a new water-soluble photoinitiator that is active and efficient in curing an aqueous lens formulation in wavelengths from 390 to 500 nm and for making UV-absorbing contact lenses from an aqueous lens formulation according to the Lightstream Technology™.